Cermet resistance elements and terminal and tap connections therefor



J n 6. 1967 M. J. STRIEF ETAL ,440,

CERMET RESISTANCE ELEMENTS AND TERMINAL AND TAP v CONNECTIONS THEREFOR Filed Sept. 11, 1964 INVENTOR. MILTON .1. STRIEF LESLIE T. #EART STA LEY SCHNEIDER BY @171 United States Patent 3,324,440 CERMET RESISTANCE ELEMENTS AND TERMI- NAL AND TAP CONNECTIONS THEREFOR Milton John Strict, Riverside, Leslie Thomas Peart, Santa Ana, and Stanley Schneider, Newport Beach, Calif.,

assignors to Eeckman Instruments, Inc., a corporation of California Filed Sept. 11, 1964, Ser. No. 395,652. 6 Claims. (Cl. 338-162) The present invention relates to electrical resistance devices of the type utilizing a resistance element in the form of a thin layer or track of resistance material disposed on a non-conductive base member and is more particularly related to terminal and tap connections for such a resistance element.

The invention is primarily directed to resistance elements and devices using a resistance material which is applied as a layer or'track on a non-conductive support or base. One such layer of resistance material, suitable for such devices, is the well-known conductive plastic material, which is formed of a mixture of powdered plastic and conductive metal particles of carbon. Another such resistance material is formed of a mixture of glass and metal particles and commonly called cermet resistance material.

Cermet type resistance elements are formed from composite mixtures of finely divided glass particles and finely divided metal particles which are applied to a non-conductive base member and fired to the temperature below the melting point of the metal but sufficient to melt the glass particles and to fuse the mixture into a conglomerate mass. In these cermet mixtures, it has been found best to use noble metals which tend to resist oxidation at the temperatures required to melt the glass and to fuse the mixture.

The fused cermet mixture produces a smooth continuous track or film having a surface that is resistant to high humidity and fungus. Because of the hard smooth surface and durability of the resistance element, it is especially applicable to variable resistance devices such as potentiometers or rheostats which utilize a movable contact that must traverse over the surface of the resistance layer. A number of typical cermet resistance films and methods of making the same are described in US. Patent Nos. 2,950,995 and 2,950,996 issued in the name of Thomas M. Place, Sr., et al., and assigned to the same assignee as the present invention.

In order to conduct an electrical current to and from the resistance track, terminal members are normally attached to the resistance element at the opposite ends thereof. They are usually in intimate contact with the resistance track and extend across the entire width thereof. In continuous turn or continuous resolution potentiometers-or variable resistance devices the resistance element is usually deposited in arcuate shape over some pre determined arcuate extent, for example 350. Between the end terminal connections at opposite ends of the element there is a gap or open space of perhaps 10. ,The gap or dead space between the end terminal connections may be filled with a suitable layer of non-conductive material which permits passage of the wiper or movable electrical contact thereover as it moves from one end of the resistance material to the other. Thus, in one full rotation, the wiper traverses around the element, leaves the element to cross the non-conductive bridge area and again returns to the electrically active portion of the resistance layer.

It has been found that, as the wiper passes from the resistance element proper to the non-conductive material in the gap, an arc sometimes results due to the high current carried by the wiper as it disengages from the resistance layer. This arcing tends to erode the resistance film and to shorten the service life of the element.

Another problem associated with deposited layer or film type resistance devices is that, in the transition of the wiper to and from the resistance element to a terminal member or a tap member that may be deposited on or overlaid upon the resistance element, there is a jump-off voltage. A jump-off voltage is an immediate change in voltage over that of the terminal or tap member and is not linear with respect to the mechanical rotation or translation of the wiper. The jump-off voltage is generally created by terminals or taps which overlay the resistance element and cause the wiper to engage (or disengage) the resistance element at a point a short distance from the edge of the terminal, causing the voltage to jump from the value at the termination to that encountered by the wiper at the point where it first touches the resistance element. This jump-off voltage may be appreciable in potentiometers and, as such, produces a less precise unit.

It is an object of the present invention to provide an improved terminal or tap member for the resistance element of a variable resistance device.

It is a more specific objectof the present invention to provide an improved thin layer resistance element having a non-conductivegap therein and terminal conductors adapted to produce a minimum jump-off voltage during transition of a current carrying wiper across the gap and a substantial reduction in arcing as the wiper traverses the non-conductive gap.

Further objects and advantages of the invention will become apparent as the following description proceeds and the feature of novelty which characterizes the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

In carrying out the objects of the present invention, there is provided a resistance element in the form of a non-conductive base member, having a flat surface area thereon, upon which is deposited a resistance layer in the form af a track adapted to be traversed by a current carrying wiper. In order to conduct current through the resistance element, there are provided terminal or tap mem bers, each in the form of a first conductive strip or portion disposed beneath the resistance track and a second conductive strip or portion overlying the resistance track andadapted to be contacted by the wiper. The first strip is so disposed with respect to the second strip that the leading edge of the first strip, as the wiper approaches the terminal, is disposed beneath the wiper before the Wiper engages the second conductive strip overlying the resistance track.

For a better understanding of these and further aspects of the invention, reference may be had to the accompanying drawing in which:

FIGURE 1 is a perspective view of a resistance element embodying the structure of the present invention;

FIGURE 2 is an enlarged partial top view of the resistance element of FIGURE 1 illustrating in greater detail the input and output terminals adjacent to the bridge section;

FIGURE 3 is a cross-sectional view taken along line 33 of FIGURE 2 illustrating the end terminals and the bridge section of the variable resistance device;

, FIGURE 4 is a partial cross-sectional view taken along line 4-4 of FIGURE 1 illustrating a tap member or terminal connection for a variable resistance device; and

FIGURE 5 is a cross-sectional view of another embodiment of the invention.

Referring now to FIGURE 1 of the drawing, there is shown a resistance element of the type adapted for use in a variable resistance device. The resistance element comprises a substrate base member 2 of dielectric material which may be in the form of a circular disc or wafer. The base member is usually formed of an unglazed homogeneous, non-porous, ceramic material, such as steatite or alumina or other non-conductor material well known in the art. While the illustrated resistance device is in the form of a circular disc of the type used in a rotary single turn potentiometer or rotary variable resistor, it will be understood that the present invention is not limited to resistance devices of this particular shape and that the invenion is equally applicable to resistance devices of rectangular shape or any other suitable configuration.

In conventional practice, the substrate or base member 2 is molded, fired and then ground or lapped to provide a smooth relatively flat surface 2a for supporting a resistance film or layer 3 thereon. The resistance layer 3 forms a conductive path when an electrical current is applied to the resistance layer through electrical conductor terminals such as members 4 and 6 and tap member 7. As hereinafter used in the specification and claims, the terms conductor terminals or terminals are meant to include both end terminal connections or center tap connections commonly employed in the resistor or variable resistor field.

The resistance element or layer 3 may be formed of a suitable resistance material such as a conductive plastic material, having a carbonaceous metal dispersed throughout the fused plastic, or of a cermet resistance material comprising a non-conductive glass binder material having minute particles of noble metal or metal alloy dispersed throughout the element. Particular examples of cermet materials and their method of manufacture are fully explained in the aforementioned Place et al. Patents Nos. 2,950,995 and 2,950,996 assigned to the assignee of the present invention.

When using cermet materials, the mixture of glass and metal particles, forming the layer 3, are deposited on a substrate by any suitable operation well known in the art, such as brushing, spraying, stenciling or silk screening. After thecermet film or layer has been deposited upon the surface 2a of the base member 2, the base and layer are preferably permitted to dry in a circulating warm air for a short period and then fired in a furnace or conventional ceramic kiln. The purpose of the firing operation is to solidify the glass particles of the material into a continuous glassy phase with the metal particles being uniformly dispersed throughout the material, and without melting the metal particles or producing bubbles or blisters on the surface of the layer 3. The mixture of glass and metal particles, after fusion at a temperature below that of the metal constituent forms acontinuous element having a hard smooth glassy surface.

When the resistance element is used as a variable resistance device or potentiometer, there is provided a wiper or electrical contact member, such as the contact member 8, which is adapted to traverse the surface of the resistance element 3 to vary the output of the device. Preferably the wiper 8 is formed of a strip of spring-like conductor material, such as beryllium copper. The end of the wiper 8 in contact with the surface of the element 3 is preferably serrated into a plurality of fingers 8a (seen best in phantom lines in FIG. 2) adapted to individually contact the surface of the element.

It is desirable, in continuous turn or 360 rotational potentiometers and variable resistance devices, to apply a non-conductive bridge in the region between the terminal connections 4 and 6 to carry the wiper of the device across the gap therebetween. In cermet type resistance elements this bridge is usually formed of glass. In conductive plastic elements, the bridge section 5 may be formed of the same plastic as the element without the conductive carbon particulate material.

As Will be hereinafter described, the present invention is directed to an improved terminal connection for conducting an electrical current to the resistance element. Referring now to FIGS. 2 and 3, it will be noted that terminal member 4 is formed of first and second strips or portions 11 and 12 respectively. Similar strips or portions 11a and 12a form terminal member 6.

In the preferred embodiment of the invention, strips of metal powder paste are applied to the base member 2, before the application of the resistance layer 3, and are then fired thereon at a sufficiently high temperature to convert the metallic paste into conductive strips 11 and 11a of metal firmly attached to the base. One metallic paste used to form these strip portions is formed of a combination of metal powders comprising about 50% by weight, gold (Au), and about 40% by weight, platinum (Pt), together with about 10% by weight of a glass binder.

The resistance element or layer 3 is then applied to the base so that the end portions of the layer 3 overlay at least a portion of the strips 11 and 1111. As shown in the embodiment of FIG. 3, the strips 11 and 11a are completely imbedded or covered by the resistance layer 3 and are in intimate electrical contact therewith. When the resistance material or layer is firedto the fusion temperature, the film tends to flow to a smooth surface, but a slight ridge may appear in the surface over the strips 11 and 11a. The surface of the resistance layer may preferably be lapped to remove any ridge that may have formed.

It should be pointed out that the thickness of the resistance layer 3 and the components of the terminal members are greatly magnified in the cross-sectional FIG- URES 3-5. The thickness of the layer 3 is extremely small and the thickness of the resistance material overlaying the first strip 11 is, of course, less than the thickness of the resistance layer 3 in the remaining portions of the track.

After the resistance layer is applied over the strips 11 and 11a, additional strip portions 12 and 12a are applied over the ends of the resistance layer 3. Strips 12 and 12a may comprise similar material as that of strips 11 and 11a and may be fired at a temperature suflicient' to convert the material to a metal conductor. It is generally desiarble, however, to utilize a material for strips 12 and 12a that has a lower conversion or fusion temperature than that of the resistance element 3 and it is desirable that the material forming strips 11 and 11a have a higher conversion or fusion temperature than that of the resistance layer 3 so that the respective materials will not be destroyed during the firing operations. Thus, in practice, applicants assignee has used the aforementioned gold, platinum and glass composition for forming the strips 11 and 11a while the conductor strips 12 and 12a have been preferably formed of commercially available fired-on silver which is well known for use as terminal connections and may comprise approximately silver and 10% glass.

Strips 11 and 12 (and 11a and 12a) preferably extend at least the entire width of the track 3. As may be seen in FIG. 2 the radially outward ends of the strips 11 and 12 (and 11a and 12a extend beyond the outer edge 3b of the track 3 to provide a space for the connection of conductor leads. Also, the radially extending portions of the strips are in electrical contact so that all portions of the terminal connection are at substantially the same po tential.

It will be noted that the edge 13 (hereinafter referred to as the leading edge) of strip 11 is offset from the forward edge 14 of the strip 12. Thus the edges 13 and 14 of the respective strips are so constructed and arranged that the leading edge 13, as the wiper 8 approaches the terminal 4, is disposed beneath the contact point of the wiper before the wiper engages the edge 14 of the overlaid strip 12. In the preferred form, it is desirable that the contact points of the wiper line up directly over the edge 13 of the underlaid' strip 11 just prior to the time that the wiper engages or contacts strip 12. The potential difference between the wiper and the terminal is then reduced to a minimum, just before the curved wiper is forced to disengage from the resistance layer as it enengages strip 12. Similarly, as the wiper traverses back to the resistance element from strip 12a, it touches the sur face of the resistance layer 3 at a point above strip 11a. The above described construction reduces the jump-ft voltage to a minimum. That is, because the strips 11 and 12 are at the same potential and the amount of resistance of that resistance material between the surface of strip 11 and the wiper contact is very small as compared to the resistance encountered between the forward edge 14 of strip 12 and the first point of contact of the wiper on the resistance element there is much less jump-off voltage than would be found in the usual overlaid terminal not utilizing an offset underlaid counterpart, such as strip 11 (or 11a), as disclosed in the present invention.

Strips 12 and 12a, being directly contacted by the wiper 8, and overlaying the resistance layer in the region of the bridge section suppress any arcing that may occur as the wiper traverses from the resistance element across the nonconductive bridge section. There is no opportunity for electrical arcing between the resistance layer and the wiper because the wiper directly engages the relatively large conductive strip 12 which is between the resistance element and the non-conductive layer as the wiper traverses onto the layer 5. Because of the high conductivity of the terminal conductor, no arcing takes place and therefore, no deleterious effects to the resistance element or the terminal conductor.

It should be noted that, while the terminal strips described above are of the type that are formed from a metal paste material, it is not intended that the invention be limited to such construction. Thus, for example, the strips could be formed of thin ribbons of conductive metal, such as silver or gold, and firmly attached to the resistance element and base in some other manner. These ribbons of conductive metal could be attached to a cermet resistance layer during the firing operation of the cermet material.

Referring now to FIG. 4 there is shown a terminal connection which may be utilized for a center tap; such as the center tap 7 of FIG. 1. This tap comprises a first relatively wide strip 16 disposed beneath the resistance track 3 and a second comparatively narrow strip 17 overlaid on the resistance layer 3 above the first strip. The second strip 17 is substantially centered over the first strip so that the edges of the first strip 16 are disposed outwardly of the edges of the second strip. The width of the first strip 16 is such that a wiper 8, on traversing over the center tap, is disposed directly above the first strip 16 before it engages the second strip 17. As the wiper 8 disengages from the second strip in passing over the terminal it will be disposed directly over the first strip. It is preferable that edges 16a and 16b of the first strip be disposed directly below the point of contact of the wiper as it engages and disengages from the edges of the second strip. The first strip 16 reduces to a minimum the jump-off voltage due to the lifting of the wiper by the overlaid strip and strip 17 reduces theend resistance of the tap connection.

Referring to FIG. 5 there is shown another embodiment of the invention in which the separate strip portions of the terminal are formed from a single strip 21. The lower or underlaid portion 21a of the strip extends for a distance beneath the resistance layer 3 and the upper portion 21b is formed by folding the strip over onto the resistance layer. As in the previous arrangements the leading edge of lower portion 21a (as the wiper approaches the terminal) is disposed beneath the contact point of the wiper 8 before the wiper engages the upper portion or forward edge of the upper strip.

While in accordance with the patent statutes there has been provided what at present are considered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, the aim of the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is: 1. An electrical resistance element for a variable resistance device comprising:

a non-conductive base member having a substantially smooth surface area thereon; a track of resistance material deposited on said smooth surface area; an electrically conductive wiper adapted to traverse said track of resistance material; an electrical terminal or tap for said resistance element comprising a first conductive strip underlying at least a part of said track and a second conductive strip overlying at least a part of said track and adapted to be contacted by said wiper, said first conductive strip also underlying a portion of said second conductive strip and extending beneath said track for a distance in the direction of wiper travel greater than said second conductive strip overlies said track, said distance being sufiicient to cause the contact point of said wiper during traversing of said track to become disposed above said first con ductive strip before engagement with said second conductive strip of said terminal. 2. An electrical resistance element for a variable resistance device comprising:

a non-conductive base member having a substantially smooth surface area thereon; a track of resistance material deposited on said smooth surface area; an electrically conductive wiper adapted to traverse said track of resistance material; an electrical terminal or tap for said resistance element comprising a first conductive strip underlying at least a portion of said track and a second conductive strip overlying at least a portion of said track and adapted to be contacted by said wiper, said first conductive strip also underlying a portion of said second conductive strip and extending beneath said track for a distance in the direction of wiper travel greater than said second conductive strip overlies said track, said distance being sufiicient to cause the contact point of said wiper during traversing said track to become disposed above said first conductive strip before engagement with said second conductive strip; and means electrically connecting said first and second conductive strips to the same source of electrical potential. 3. An electrical resistance element for variable resistance device comprising: I a non-conductive base member having a substantially smooth surface area thereon; a track of resistance material deposited on said smooth surface area; an electrically conductive wiper adapted to traverse said track of resistance material; an electrical tap for said resistance element comprising a first conductive strip underlying at least a por tion of said track and a second conductive strip overlying at least a portion of said track, and adapted to be contacted by said wiper, said first conductive strip being substantially wider than said second conductive strip with said second conductive strip centered above said first conductive strip said first conductive strip being of greater width than said second conductive strip so that opposite edges of said first conductive strip extend beneath said track for a distance in the direction of wiper travel greater than said second conductive strip overlies said conductive track so that the contact point of said wiper during traversing of said track is disposed above said first conductive strip before engagement and after disengagement with said second conductive strip.

4. An electrical resistance element for a variable re sistance device comprising:

a non-conductive base member having a substantially smooth surface area thereon;

a track of resistance material deposited on said smooth surface area in a configuration having a gaptherein;

an electrically conductive wiper adapted to traverse the surface of said track of resistance material;

a pair of electrical terminals disposed on opposite sides of said gap in electrical contact with said element said terminals each comprising a first strip of elec trically conductive material underlying at least a portion of said resistance track adjacent one edge of said gap, and a second strip of conductive material overlying at least a portion of said track adjacent said gap and adapted to be contacted by said wiper, said first conductive strip also underlying a portion of said second conductive strip and extending beneath said resistance track in the direction of wiper travel for a distance greater than said second strip overlies said resistance track, said distance being sufficient to cause the contact point of said Wiper as it approaches said terminal from said resistance track to become disposed above said first conductive strip before engagement with second conductive strip; and

a non-conductive bridge material in said gap for transferring the wiper across the gap,

5. An electrical resistance element for a variable resistance device comprising:

a non-conductive base member having a substantially smooth surface area thereon;

a track of resistance material deposited on said smooth surface area;

an electrically conductive wiper having a curved surface a portion of which is in contact with the track of resistance material;

an electrical terminal for said resistance element comprising a first conductive strip disposed normal to said track and underlying at least a portion of said track; and

a second conductive strip disposed normal to said track and overlying at least a portion of said track and adapted to be contacted by said Wiper, said first conductive strip also underlying a portion of said second conductive strip and extending beneath said track in the direction of wiper travel for a distance greater than said second conductive strip overlies said conductive track, said distance being equal to at least the distance subscribed beneath said wiper between the point of wiper contact and the point said wiper first engages said second strip.

6. An electrical resistance element for a variable resistance device comprising:

a non-conductive base member having a substantially smooth surface area thereon;

a track of resistance material deposited on said smooth surface area; I

an electrically conductive wiper adapted to traverse the surface of said track of resistance material;

an electrical terminal or tap for said resistance element comprising a strip of electrically conductive material folded around an edge of said resistance element so that at least a portion of said strip underlies said resistance element and a portion thereof overlies said resistance element, said portion of said strip underlying said resistance element extending beneath said track in the direction of wiper travel for greater distance than said portion of said strip overlying said conductive track, said distance being sufiicient to cause the contact point of said Wiper during traversing of said track to become disposed over said portion of said conductive strip beneath said track before engagement with said portion of said conductive strip overlying said track.

References Cited UNITED STATES PATENTS 1,881,446 10/1932 Flanzer. 2,463,045 3/1949 Mucher 338-439 X 2,720,572 10/1955 Moore 338-164 X RICHARD M. WOOD; Primary Examiner.

J. G. SMITH, Assistant Examiner. 

1. AN ELECTRICAL RESISTANCE ELEMENT FOR A VARIABLE RESISTANCE DEVICE COMPRISING: A NON-CONDUCTIVE BASE MEMBER HAVING A SUBSTANTIALLY SMOOTH SURFACE AREA THEREON; A TRACK OF RESISTANCE MATERIAL DEPOSITED ON SAID SMOOTH SURFACE AREA; AN ELECTRICALLY CONDUCTIVE WIPER ADAPTED TO TRAVERSE SAID TRACK OF RESISTANCE MATERIAL; AN ELECTRICAL TERMINAL OR TAP FOR SAID RESISTANCE ELEMENT COMPRISING A FIRST CONDUCTIVE STRIP UNDERLYING AT LEAST A PART OF SAID TRACK AND A SECOND CONDUCTIVE STRIP OVERLYING AT LEAST A PART OF SAID TRACK AND ADAPTED TO BE CONTACTED BY SAID WIPER, SAID FIRST CONDUCTIVE STRIP ALSO UNDERLYING A PORTION OF SAID SECOND CONDUCTIVE STRIP AND EXTENDING BENEATH SAID 